Maintaining connectivity: understanding the role of root order and mycelial networks in fine root decomposition of woody plants

The predictive power of climate models is limited by an incomplete understanding of the controls on fine root decomposition and thus belowground carbon cycling. To more accurately model rates of decay, fine root heterogeneity needs to be addressed in fine root decomposition studies. Branching order integrates both structural and chemical properties that are important in indicating litter quality and decay rate. We discuss current views on the controls and patterns of fine root decomposition in combination with recent findings related to the effects of branching order and mycorrhizal decomposition. We examine the counterintuitive finding that nitrogen rich, lower order roots decompose more slowly than woody, higher order roots in temperate and sub-tropical forests. We posit that slower decomposition of first and second compared to higher order roots might be caused by the poor carbon quality associated with higher concentrations of phenols in lower order roots or by inhibition of saprophytes by the mycorrhizal fungi that often preferentially inhabit these roots. Alternatively, apparent recalcitrance of lower order roots could be an experimental artifact caused by severing pre-mortem mycelial connections during sample processing, or exclusion of animals that graze fungal structures by the small mesh sizes characteristic of litterbags. To better predict the residence time of the carbon contained in the entire fine root pool, existing methods should be applied to individual root orders when practical. New methods for characterizing decomposition of undisturbed roots that have senesced naturally are greatly needed.